This relates generally to imaging devices, and more particularly, to imaging devices having backside illuminated image sensors with redistribution layers.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. Conventional image sensors are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology. The image sensors may include photodiodes and other operational circuitry such as transistors formed in a front surface of the substrate. A dielectric stack is formed on the front surface of the substrate directly on top of the photodiodes. The dielectric stack includes metal routing lines and metal vias formed in dielectric material.
A color filter array is formed over the dielectric stack to provide each pixel with sensitivity to a certain range of wavelengths. Microlenses may be formed over the color filter array. Light enters from a front side of the image sensor (i.e., light enters the microlenses and travels through the color filters into the dielectric stack). An image sensor used in this way is referred to as a frontside illumination (FSI) image sensor. Because the light must pass through the metal routing lines and metal vias of the dielectric stack in an FSI image sensor, internal reflections within the dielectric stack may cause cross-talk between neighboring image sensors. The size of photosensitive elements in an FSI image sensor is limited due to the space required for routing lines, etc. in the dielectric stack in front of the photosensitive elements.
To address these issues, backside illumination (BSI) image sensors have been developed. In conventional BSI image sensors, microlenses may be formed on the back surface of the substrate on the opposite side of the photodiodes from the dielectric stack. In a typical arrangement, a color filter array is formed under the microlenses on the back surface of the substrate to provide each pixel with sensitivity to a certain range of wavelengths. Light enters from the back side of the image sensor (i.e., light enters the microlenses and travels through the color filters onto the photodiodes).
In a conventional BSI image sensor, metal routing lines are coupled to bond pads on the front side of the dielectric stack. Bond pads are coupled to external circuitry with wire bonds. Lower cost, higher efficiency manufacturing and compact packaging of BSI imager sensors may be achieved by coupling bond pads to external circuitry using solder balls attached to the front surface of a thinned semiconductor substrate in a wafer level packaging process, prior to singulation of the wafer into individual die.
It would therefore be desirable to provide a method of forming a ball grid array on the front side of a BSI image sensor capable of handing thinned semiconductor substrates for wafer level packaging.